This invention relates generally to electronic circuitry and, more particularly, to electronic circuitry adapted to multiply/divide analog signals.
As is known in the art, electronic circuitry adapted to multiply/divide analog signals has a wide variety of applications. One such circuit, a so-called "log-analog multiplier," includes four transistors having serially coupled base-emitter junctions. The output current produced in a fourth, or output, one of the four transistors is proportional, to an approximation, to the product of the collector currents in a pair of such transistors divided by the collector current of the third transistor. The pair of transistors and the third transistor are coupled in the feedback path of a corresponding one of three operational amplifiers. Generally, the output of any one of such operational amplifiers is connected to the emitter electrode of the corresponding transistor through a resistor, and the input to such operational amplifier is connected to the collector electrode of such transistor. If the operational amplifier is assumed to have zero offset current and voltage, then the input current to the operational amplifier will pass to the emitter electrode of such transistor (assuming such transistor has a high beta (ratio of collector current to base current)). That is, the collector current will be approximately equal to the input current, and the output voltage of the operational amplifier will be proportional, to an approximation, to the natural log of the collector current and, hence, to the natural log of the input current. Because of the reactive characteristic of the transistor, it is generally necessary to provide a capacitor between the collector electrode of the transistor and the output of the operational amplifier for stabilization. The use of such capacitor in the feedback path for stabilization reduces the bandwidth, and hence time response, of the circuit.